DNA-RNA and RNA-RNA hybridization are important to many aspects of nucleic acid function including DNA replication, transcription, and translation. Hybridization is also central to a variety of technologies that either detect a particular nucleic acid or alter its expression. Antisense nucleotides, for example, disrupt gene expression by hybridizing to target RNA, thereby interfering with RNA splicing, transcription, translation, and replication. Antisense DNA has the added feature that DNA-RNA hybrids serve as a substrate for digestion by ribonuclease H, an activity that is present in most cell types. Antisense molecules can be delivered into cells, as is the case for oligodeoxynucleotides (ODNs), or they can be expressed from endogenous genes as RNA molecules. The FDA recently approved an antisense drug, VITRAVENE™ (for treatment of cytomegalovirus retinitis), reflecting that antisense has therapeutic utility.
Enhanced red blood cell production mediated by the hormone erythropoietin (EPO) is a well-known adaptive response of humans to hypoxia (Bunn et al. (1996) Physiological Rev. 76, 839-845). Normally produced by the adult kidney and the fetal liver, EPO stimulates the development of EPO receptor (EpoR) expressing red blood cell precursors in the bone marrow.
EPO is a glycoprotein (46 kDa) hormone produced by renal interstitial cells within the kidney that regulate the production of red blood cells in the marrow. These cells are sensitive to low arterial oxygen concentration and will release erythropoietin when oxygen is low (hypoxia). Erythropoietin stimulates the bone marrow to produce more red blood cells thereby increasing the oxygen carrying capacity of the blood. EPO exerts its effect by binding to EpoR on the surface of erythrocyte precursors in the bone marrow. EPO expression, however, has been observed on the cells from other normal tissues in addition to renal interstitial cells including enterocytes, trophoblast and neuronal cells.
The measurement of EPO in blood stream can indicate bone marrow disorders or kidney disease. Normal levels of erythropoietin are 0 to 19 mU/ml (milliunits per milliliter). Elevated levels can be seen in polycythemia rubra vera, a condition characterized by enlargement of spleen and increased production of red blood cells by bone marrow. Lower than normal values are seen in chronic renal failure leading to anemia. Chronic renal failure leads to anemia, in part, because of the progressive absence of adequate EPO production for the maintenance of erythropoiesis.
Hypoxia is predominant feature of solid tumors, which comprise approximately ninety percent of all human cancers. Adaptive responses to hypoxia in solid tumors has been correlated with enhanced aggressiveness, reduced tumor cell death and diminished tumor response to both radiation and chemotherapy. EPO expression has been observed in different hematopoietic and non-hematopoietic malignancies and has been shown to mediate autonomous growth of erythrocytic leukemia cells expressing EpoR.
Many common human cancers over-express the hypoxia-inducible transcription factor HIF-1, which regulates the expression of EPO as well as several genes required for enhancing hypoxic survival of cancer cells including genes coding for glycolytic enzymes, glucose transporters, and vascular endothelial growth factor (Semenza (1999) Ann. Rev. Cell. Dev. Biol. 15,551-578). Tumor hypoxia is recognized as a major factor in the tumor resistance to chemotherapy and radiation therapy although the underlying mechanisms are unknown. Hypoxia induces adaptive responses in cells largely by activating the expression of several genes under the regulation of hypoxia-inducible factor-1 (HIF-1), a heterodimeric transcription factor composed of HIF-1α and HIF-1β subunits.